The Arizona Department of Environmental Quality (ADEQ) is working on four projects to improve surface water quality around historic mining locations, and one of them is in the Pinal Mountains a few miles southwest of Miami.
In the early 1900s the Gibson Mine, near the now-ghost town of Bellevue, was one of the area’s prominent copper producers – in the spring of 1910, according to the Daily Arizona Silver Belt, the mine was sending about 25 tons of high-grade ore to the Old Dominion smelter a week. The site was mined off and on from the 1960s through the 1990s. Since then the Gibson has been inactive, but it left a legacy of copper concentrations affecting the water in nearby Pinto Creek – and ADEQ’s current project is meant to further address that legacy.
It is not the first remediation work at the Gibson Mine, which a 2001 Environmental Protection Agency study identified as a major source of copper exceedances. Remediation projects in 2007 and 2009 brought down copper concentrations by approximately 85%, but mine runoff was still degrading the creek’s water quality. Starting in 2017, ADEQ has been taking additional water and soil samples to help determine further measures; the majority of their sampling is now complete. “Our goal for the Gibson Mine project is to further reduce copper concentrations in Pinto Creek to meet the surface water quality standard established to protect wildlife,” said ADEQ Communications Director Caroline Oppleman.
In January 2021 ADEQ awarded a contract for the additional remediation work, which is slated to begin this October, to Arcadis, a global design and consulting firm with offices in the Phoenix area. The project involves several phases, most of which are finished; water and soil sampling, developing a safety plan, data collection and analysis, and remediation design. Permits are currently being obtained, including a federal Clean Water Act 404 permit from the Army Corps of Engineers, Clean Water Act 401 state certification, State Historic Preservation Act cultural clearances, and biological clearances under the Endangered Species Act. Once all permits are in place, the remediation work is expected to take a few months to complete.
The work will include excavating soil with high copper levels from drainages leading to the tributary, and installing riprap in the channels to keep water clean during storm flows and reroute storm flow to bypass waste rock and tailings. ADEQ will also consolidate the excavated soil into a stockpile with the remaining waste rock and tailings, then regrade, cover and re-seed it to encourage vegetation growth and prevent future erosion. After project completion, further water and soil testing will be conducted to verify a successful remediation.
The project area is on 230 acres of private land, donated to the Franciscan Friars of California in 1969, and a small section of public land. The Friars have been working with ADEQ and the U.S. Forest Service since 2006 to reduce copper levels in Pinto Creek. “ADEQ appreciates the Friars’ commitment to partnering with us to conduct this important work to improve surface water quality,” said Oppleman.